"coming out of the cupboard" is a figure of speech for popular or non popular people's disclosure of their secret obsession for Harry Potter.
The term comes from the well know phrase "coming out of the closet" which refers to a person revealing that they are gay/lesbian.
The "closet" part is replaced by "cupboard" because, as we know, Harry Potter lived in the "cupboard under the stairs" in the beginning of the book series.

I guess I'm finally coming out of the cupboard. I didn't really want people to know I was secretly obsessed with Harry Potter